Balanced means the total number of 1s and 0s are the same and when reading from high to low has count(1s) >= count(0s) at all times, which is to say any closing ")" must have a preceding matching open "(".

Because the number of 1s and 0s are equal the width is always an even 2*w. The number of values with a given width is the Catalan number C(w) = (2w)!/(w!*(w+1)!). For example 6-bit values w=6/2=3 gives C(3) = (2*3)!/(3!*4!) = 5 many such values, being i=4 through i=8 inclusive as shown above. (See Math::NumSeq::Catalan.)

Binary Trees

The sequence values correspond to binary trees where each node can have a left and/or right child. Such a tree can be encoded by writing

0 if no node (empty tree)
1,left-tree,right-tree at a node

The "left-tree" and "right-tree" parts are the left and right legs of the node written out recursively. If those legs are both empty (ie. the node is a leaf) then they're empty trees and are "0" giving node "100". Otherwise the node is 1 followed by various more 1s and 0s. For example,

d
/
b c => 11001010 [0]
\ / ab c d ^-final zero of encoding omitted
a

At "a" write 1 and recurse to write its left then right legs. The left leg is "b" so write 1 and the two legs of "b" are empty so write 0,0. That completes the left side of "a" so resume at the right side of "a" which is 1 for "c" and descend to the left and right of "c". The left of "c" is empty so write 0. The right of "c" is "d" so write 1 and the two empty legs of "d" are 0,0. The very final 0 from that right-most leaf "d" is dropped (shown "[0]" above).

This encoding can also be applied breadth-first by pushing the left and right descents onto a queue of pending work instead of onto a stack by recursing. In both cases there's an extra final 0 which is dropped. This 0 arises because in any binary tree with K nodes there are K+1 empty legs. That would give K many 1-bits and K+1 many 0-bits.

In this encoding the balanced binary condition "count 1s >= count 0s" corresponds to there being at least one unfinished node at any time in the traversal (by whichever node order).

The NumSeq code here acts on values as numbers. Tree encodings like this are probably better handled as a string or list of bits.

Mountain Ranges

A cute interpretation of the opens and closes is as up and down slopes of a mountain range. 1-bit for up, 0-bit for down. For example,

/\
/ \ /\
/ \/ \
/ \/\
----------------
11110001100010

The mountain range must end at its starting level and must remain at or above its starting level at all times.

Numerical order of the values means narrower mountain ranges are before wider ones, and two ranges with equal width are ordered by down-slope preceding up-slope at the first place they differ.

Random Access

If $value is balanced binary then return its index i. If $value is not balanced binary then return undef.

$i = $seq->value_to_i_ceil($value)

$i = $seq->value_to_i_floor($value)

Return the index i of $value or if $value is not a balanced binary integer then return the i of the next higher or lower value, respectively.

FORMULAS

Next

When stepping to the next value the number of 1s and 0s does not change. The 1s move to make a numerically higher value. The simplest case is an isolated low 1-bit. It must move up one place. For example,

11100100 isolated low 1-bit
-> shifts up
11101000

If the low 1 has a 1 above it then that bit must move up and the lower one goes to the low end of the value. For example

1110011000 pair of bits
-> one shifts up, other drops to low end
1110100010

In general the lowest run of 1-bits is changed to have the highest of them move up one place and the rest move down to be a ...101010 pattern at the low end. For example a low run of 3 bits

1111100111000000 run of bits
-> one shifts up, rest drop to low end
1111101000001010
^ ^ ^
up low end

The final value in a 2*w block has all 1s at the high end. The first of the next bigger block of values is an alternating 1010..10. For example

111000 last 6-bit value, all 1-bits at high end
->
10101010 first 8-bit value

Ith

As described above there are Catalan(w) many values with 2*w bits. The width of the i'th value can be found by successively subtracting C(1), C(2), etc until reaching a remainder i < C(w). At that point the value is 2*w many bits, being w many "1"s and w many "0"s.

In general after outputting some bits of the value (at the high end) there will be a number z many "0"s and n many "1"s yet to be output. The choice is then to output either 0 or 1 and reduce z or n accordingly.

numvalues(z,n) = number of sequences of z "0"s and n "1"s
with remaining 1s >= remaining 0s at all times
N = numvalues(z-1,n)
= how many combinations starting with zero "0..."
if i < N then output 0
if i >= N then output 1 and subtract N from i
(which is the "0..." combinations skipped)

The last entry numvalues(w,w) in each row is Catalan(w), so that can be used for the initial i subtractions seeking the width w. If building or extending a table each time then stop the table at that point.

Catalan(w) grows as a little less than a power 4^w so the table has a little more than log4(i) many rows.

SEE ALSO

HOME PAGE

LICENSE

Copyright 2012, 2013, 2014 Kevin Ryde

Math-NumSeq is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3, or (at your option) any later version.

Math-NumSeq is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with Math-NumSeq. If not, see <http://www.gnu.org/licenses/>.

Module Install Instructions

To install Math::NumSeq::BalancedBinary, simply copy and paste either of the commands in to your terminal